It has heretofore been a conventional practice to incorporate solid glass particles in certain plastics as filler material. However, there are drawbacks to this practice. Such solid glass structures are typically on the order of 2.3-2.6 g/cc, which is undesirably dense for many applications, such as those in which weight-minimization is a desideratum. Also, use of solid glass particles as filler material in plastics imparts an unattractive greyish off-color to those plastics.
As an alternative to the use of solid glass particles as filler material, the art has proposed the use of low density glassy particles as fillers. The approach generalIy taen in providing such lightweight particles is production of hollow glass spheres, commonly known as microballoons. Conventional microballoon technology generally involves the utilization of glass particles and a substance referred to as a blowing agent. The object is to obtain molten glass by heating the particles and then, by releasing the blowing agent, expand the molten glass into glass particles having a hollow interior (possibly containing gas at a pressure higher than atmospheric). Examples of patents directed to such embodiments are Beck et al. U.S. Pat. No. 3,365,315, granted Jan. 23, 1968, and Taupin et al. U.S. Pat. No. 4,234,330, granted Nov. 18, 1980. Alternatively, it has also been suggested to combine glass-forming materials and a blowing agent (or substances which upon reaction generate a blowing agent), and then effect simultaneous glass formation and triggering of the release of the blowing agent to cause expansion of the molten glass after its formation. Examples of patents relating to these latter embodiments are Veatch et al. U.S. Pat. No. 2,797,201, granted June 25, 1957, Veatch et al. U.S. Pat. No. 2,978,339, granted Apr. 4, 1961, Veatch et al. U.S. Pat. No. 2,978,340, granted Apr. 4, 1961, Veatch et al. U.S. Pat. No. 3,030,215, granted Apr. 17, 1962, Veatch et al. U.S. Pat. No. 3,129,086, granted Apr. 14, 1964, DeVos U.S. Pat. No. 4,059,423, granted Nov. 22, 1977 and DeVos et al. U.S. Pat. No. 4,063,916, granted Dec. 20, 1977.
However, conventional microballoon techniques have serious shortcomings. It is impractically difficult to obtain microballoons which have a density sufficiently high and sufficiently uniform (from particle to particle) to make them commercially useful for incorporation in thermosetting plastics or thermoplastics. This is the result of the high degree of difficu1ty entailed in attempting to control the action of the blowing agent. More specifically, the problem arises because the blowing agent has the tendency to expand within the particles of molten glass until their walls become so thin that the blowing agent passes through by diffusion. This causes the particles to have densities which are normally in the range of 0.1 g/cc to 0.7 g/cc. Although densities up to 1.0 g/cc have apparently been attained using conventional technology, it is not clear whether those densities can be reproducibly obtained; in any event, the difficulty involved in attaining densities of that magnitude would, it stands to reason, increase the cost of manufacture disadvantageously. As examples of the manner in which conventional microballoons are deficient, it is noted that (a) because their density is less than that of the typical density of the liquid reaction mixture used to make cast thermosetting plastics, conventional microballoons have a tendency to separate out and float in that liquid thereby preventing a uniform distribution of the filler particles in the ultimately cast product, and (b) the thin-walled structure of the conventional microballoons causes them to be relatively low in strength, which results in their collapse or being crushed or broken when included in thermoplastics that are subjected to the high pressures and shearing forces exerted in extruding equipment, injection molding equipment and compounding equipment.
It is a further disadvantage of conventional microballoons that their chemical compositions are often limited within a narrow range as a prerequisite of practicing such technology. Consequently, it is often the case that substances which are damaging to plastics in which the conventional microballoons are being incorporated must nevertheless be employed because of the compositional constraints imposed in order to make the microballoons.